It seems that it’s easy to discern the difficulty mode of a boss kill screenshot, just by looking at it.

Mythic: Everyone lined up, various angles taken.

Heroic: Most people there, a few random toys popped

Normal: Half the raid gone as soon as it’s dead, others sprinting for the portal, and the insta pings of people leaving discord.

So, yeah, this was our Normal G’huun kill…*tries to remember the date*…